This invention relates to titanium-alloy articles that are subject to alpha case formation and, more particularly, to the heat treatment of such articles to reduce the incidence of alpha case formation.
Titanium-alloy articles may require heat treatment, after they are processed to substantially their final shapes and dimensions. For example, titanium-alloy gas turbine parts may require a final heat treatment to stress relieve the parts after forging, rejuvenation, or repair operations, or for mid-service stress relief.
Some titanium alloys, such as near-alpha, alpha, near-beta, beta, and alpha-beta titanium alloys, are subject to the formation of an alpha-embrittled zone at the surface of the article during heat treatment at a sufficiently high temperature and for a sufficiently long time in the presence of oxygen gas. The alpha-embrittled zone of oxygen-enriched alpha phase is generally termed an xe2x80x9calpha casexe2x80x9d. The alpha case is deleterious to the subsequent use of the article in some applications, because it has reduced fatigue resistance and increased susceptibility to impact damage, as compared with the underlying alpha-beta or other microstructure. When an alpha case is formed at the surface of a titanium-alloy gas turbine compressor blade, for example, it becomes susceptible to fatigue failure and also to impact failure by foreign objects ingested into the compressor.
The formation of alpha case limits the heat treatment of titanium alloys according to the composition of the alloy and the time and temperature of the heat treatment. As an example, the titanium alloy Ti-442 (Ti-4Al-4Mo-2Sn0.5Si) alloy is limited to a maximum heat treatment of about 1100xc2x0 F. for 2 hours in vacuum in most circumstances by the formation of alpha case. For some processing methods now in development, higher temperatures and longer heat treatment times are required. However, if the heat treatment is at such higher temperatures or for longer times, an unacceptable thickness of alpha case forms. The alpha case may be removed by a chemical etching process, but the chemical etching is slow and adds substantially to the cost of the article. Chemical etching is not feasible for repairs or other situations where the part is already at its specified final dimension, because the chemical etching removes metal and may reduce the dimensions to below their acceptable range.
The heat-treating conditions that avoid the formation of an excessive alpha case on susceptible articles are known under some heat treatment conditions. However, it is difficult to extend the operable practices to other conditions. In a common example, experience has shown that researchers in the laboratory are often able to develop operable heat treatments to avoid the formation of excessive alpha case, but that these laboratory heat treatments cannot be successfully applied in many production settings. Similarly, operable practices developed for one production heat treatment furnace cannot be readily extended to another production heat treatment furnace.
There is therefore a need for an approach to limit the thickness of alpha case formation on titanium alloys having such a susceptibility that may be widely applied to different heat treatment conditions. The approach must be operable both for laboratory-scale work and also for production-scale operations. The present invention fulfills this need, and further provides related advantages.
The present approach provides a method for heating treating titanium-alloy articles that are susceptible to the formation of an alpha case. The method may be applied in both a laboratory and in a production environment in a manner that permits the laboratory results to be used in production heat treatment operations or procedures successful in one production setting to be applied in another production setting. The alpha case may be eliminated entirely, or it may be limited as needed consistent with other processing operations. The approach reliably produces the expected result.
A method for heat treating titanium-alloy articles comprises the steps of first determining, for a first set of titanium articles in a first vacuum furnace and for a first set of heat treatment conditions, a minimum surface area of the first set of titanium articles associated with an acceptable alpha case formation for the first set of titanium articles, and second determining, for a second set of titanium articles in a second vacuum furnace and for a second set of heat treatment conditions, a minimum surface area of a second set of titanium articles associated with an acceptable alpha case formation for the second set of titanium articles, responsive to the value of the minimum surface area of the first set of titanium articles. A third set of titanium articles is thereafter heat treated in the second vacuum furnace and for the second set of heat treatment conditions, where the surface area of the third set of titanium articles is not less than the value of the minimum surface area of the second set of titanium articles.
In a case of interest, the second vacuum furnace is different from the first vacuum furnace. That is, the first vacuum furnace may be a first production vacuum furnace, and the second vacuum furnace is a second production vacuum furnace. Or the first vacuum furnace may be a laboratory vacuum furnace, and the second vacuum furnace is a production vacuum furnace. Typically with this approach, the second set of heat treatment conditions is the same as the first set of heat treatment conditions.
The present approach desirably utilizes a figure of merit to determine the minimum surface areas of the sets of titanium articles. The preferred figure-of-merit approach incorporates an effective pumping rate of a vacuum furnace, a surface area of the set of titanium articles, a real leak rate of a vacuum furnace, and an outgassing leak rate of the vacuum furnace. A most preferred form of the figure of merit is
FOM=(Peff+Kxc2x7ATi)/(RL+VL). 
FOM is a figure of merit value, Peff is an effective pumping rate of a vacuum furnace, Kxc2x7ATi is a self-pumping rate of the set of titanium articles in the vacuum furnace, K is a self-pumping constant, A is a surface area of the set of titanium articles, RL is a real leak rate of the vacuum furnace, and VL is an outgassing leak rate of the vacuum furnace. All of the pressures and rates are normally specified in terms of the oxygen partial pressure, but they may be expressed in terms of the total pressure if the percentage of oxygen stays constant as it does in the usual case when the only leaking and outgassing gas is air.
Thus, a method for heat treating titanium-alloy articles that are subject to the formation of an alpha case comprises the steps of first determining, for a first set of titanium articles in a first vacuum furnace and for a first set of heat treatment conditions, a value of FOM associated with an acceptable alpha case formation for the first set of titanium articles, for a relationship
FOM=(P1eff+Kxc2x7A1Ti)/(RL1+VL1), 
wherein P1eff is an effective pumping rate, Kxc2x7A1Ti is a self-pumping rate of a first set of titanium articles, K is a self-pumping constant, A1 is a surface area of the first set of titanium articles, RL1 is a real leak rate, and VL1 is an outgassing leak rate. There is a second step of determining, for a second set of titanium articles in a second vacuum furnace and for a second set of heat treatment conditions, a value of A2Ti for a relationship
A2Ti=1/K[FOMxc2x7(RL2+VL2)xe2x88x92P2eff], 
wherein A2Ti is a minimum permitted surface area of the second set of titanium articles, P2eff is an effective pumping rate, RL2 is a real leak rate, and VL2 is an outgassing leak rate. A third set of titanium articles is heat treated in the second vacuum furnace and for the second set of heat treatment conditions, where the surface area of the third set of titanium articles is not less than A2Ti.
Typically but not necessarily, the second vacuum furnace is different from the first vacuum furnace, and the second set of heat treatment conditions is the same as the first set of heat treatment conditions.
In the usual approach, the step of first determining includes first measuring P1eff, K, RL1, and VL1, and then determining FOM for the first vacuum furnace. The step of second determining then includes second measuring K, P2eff, RL2, and VL2 for the second vacuum furnace, and then calculating A2Ti from these measurements and the FOM obtained from the step of first determining. The determination of FOM in the first determining step may be made either for a single point defining an acceptable alpha case thickness, or in a parametric fashion so that FOM is related to the surface areas of the different sets of titanium articles or other controllable variable.
The present approach provides a reliable technique for heat treating titanium-alloy articles that are otherwise susceptible to the formation of alpha case, and a tool for establishing heat treatments under various conditions. The approach allows the alpha case to be avoided entirely, or restricted in its formation to a selected value consistent with the required properties and/or with production techniques that may be applied to remove the alpha case. Successful procedures developed in one context may be extended, in a suitably modified form, to other contexts. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.